This invention relates to a cylindrical composite shaft which includes end pieces to transmit torsional forces. Currently it is well known that open ended, generally cylindrically shaped tubes formed of composite materials, such as materials comprising graphite, boron, aramid, glass fibers and epoxy, polyester and/or vinylester matrix, have extremely high strength to weight ratios. Also, depending on composite material selection, these tubes are capable of high stiffness to weight ratios and are consequently replacing metal tubes for the transmission of tensile, compression, bending and/or torsional loads. Some common applications of composite material tubular shafts are rollers in paper mills, bicycle frames, driveshafts, and golf club shafts. Composite driveshafts also have another well-known advantage over all-metal shaft constructions by providing a major reduction in noise, vibration, and harmonics. All composite materials have natural dampening characteristics. Perturbations due to the exterior environment or connections (drivetrain, etc.) are absorbed by the composite material, instead of transferring the perturbations to the next component of the vehicle. However, widespread use of composite material shafts for has been extremely limited as a result of structural and economic limitations associated with the design and construction of the shafts.
Structural limitations of common practice occurs when highly loaded shaft applications are necessary. It is well known in the art of composite shaft construction to bond metal end pieces into the open ends of the tubular shaft, formed by a composite tube, to transmit forces into the tube from the adjacent mechanism or structure, this defines a composite shaft which is a composite tube plus end pieces. Successful load transfer from the metallic end pieces to the composite tube usually relies on a single load path, which is defined by the bonding material adhesive between the end piece and composite tube surfaces. While this practice is accepted in the aerospace industry, it creates many doubts by potential customers outside of aerospace, especially in the area of high load applications such as driveshafts. From a structural standpoint, these composite shafts are totally dependent upon the quality and inherent characteristics of the bonding process, as well as being subject to potential human error which may be introduced during assembly and bonding of the end pieces to the composite tube.
Economic limitations associated with common practice for constructing composite shafts generally result from the high labor content of the manufacturing processes, and in particular the high labor content associated with construction of the composite tube construction, and the high labor content associated with the insertion and bonding process for attaching the metallic end pieces into the composite tube.
With regard to the cost associated with constructing the composite tube, it is typical or common practice during the manufacturing process of composite tubes to utilize a reusable metal tool. This metal tool is typically a high-grade steel which requires a high level surface finish resulting in a fairly expensive tool. Uncured wet composite material (fiber plus resin matrix) is wrapped, pultruded or wound on the metal tool outside diameter after which the composite material is cured at a predetermined temperature for an established period time, whereby the composite tube is formed with a final shape corresponding to the shape of the metal tool. Finally, the metal tool is pressed, i.e., extracted, from the composite tube inside diameter. This reusable metal tool is then cleaned and prepared, including for example application of a release agent, between each composite tube fabrication cycle. This tool cleaning and preparation portion of the manufacturing process requires considerable time and attention to detail which ultimately adds substantial cost to the final part. In addition, it is also common practice to include a machining step in preparation for receiving the end pieces wherein the open ends of the cured composite tube are faced off.
With regard to the cost associated with insertion and bonding of the end pieces into the composite tube ends, the operations associated with this process are considered critical to the proper performance of the final product and therefore requires substantial time and attention to detail. In particular, this process is responsible for the performance of the entire shaft structure, in that the bonding material provides a single load path between end pieces and the composite tube. Quality concerns and repeatability of this process is of the utmost importance. Failure to follow any part of this critical process may result in catastrophic part failure as a result of the end piece debonding from the composite tube. Typical in the art of this process is the critical preparation of the end piece surface and composite tube surface. The surfaces which create the bond layer interface must be extremely clean and free of all oil and other contaminants. If this does not happen, the bonding agent will not adhere to the surfaces substrate resulting in an adherend failure. Consequently, human error plays a large roll in any failures resulting from defects in the bonding process. Obviously, due to the criticality of this process, strict manufacturing process and quality procedures must be followed (typical of aerospace industry), which results in high labor content. Obviously this process, while adequate for certain lightly loaded high cost applications does not lend itself to high volume production.
One approach that has been attempted to eliminate constructions requiring a tool extraction step is the use of plastic foam core material over which the composite material is wrapped. This foam core remains part of the cured shaft and of course is cylindrical in shape. However, great difficulties arise in the manufacturing process due to the low mechanical properties of the foam core itself. Alignment of end pieces and adequate strength and stiffness required for the manufacturing process limits the success of this approach, especially for long narrow shafts with high length to diameter ratios.
Clearly, in order to fully utilize the superior mechanical properties, including a high strength-to-weight ratio of composite material tubes and move composite shafts from very limited use, such as in high cost/low load applications, to the mass produced widespread applications, an improved design and process for manufacturing composite shafts is needed to address structural issues and to reduce manufacturing cost.